Since leaving conditioner in my hair, it stays moisturized for longer periods of time. The leave-in acts as a gentle base of moisture and the heavy butters, creams, and oils I use seal in that moisture. As far as the feel in your hair, I think it depends on the conditioner. When I first went natural I was using Suave Humectant, Suave Healthy Curls, Pantene Hydrating Curls, HE Hello Hydration etc. to leave in my hair because they made great detanglers. Yet, they would leave my hair feeling like plastic.
I soon found out that like moisturizers, you have to be particular about which conditioners you leave in your hair. I now choose my conditioners based on what ingredients I favor in my moisturizers. If you tend to like moisturizers with certain oils or extra ingredients (glycerin, rosemary oil, yarrow extract) then look for that in the conditioner you leave-in. All the conditioners that I mentioned above left my hair coated and crispy because they contained lots of proteins and high oil content, which my hair can't tolerate in basic moisturizers. The plasticky feeling was due to the silicones.
I now use Giovanni 50/50 and there's no stiffness, stickiness, buildup, or residue when it dries. It feels like there is a moisturizer on my hair, not hard conditioner. Some people also use the Direct-leave in (basically a conditioner marketed as a leave-in) and DevaCurl One C
. Check out the haircare guide at the top of this forum for good conditioners to leave-in. A lot of them aren't even marketed as 'leave-ins', but they work well. Definitely pay attention to the ingredients, choose what you like in your hair, and don't be afraid to splurge since you won't be rinsing it out and it will essentially be a moisturizer in your hair underneath your other products.